The present invention relates to semiconductor diode lasers. In particular, the present invention relates to semiconductor diode lasers with asymmetric bandgaps and refractive indices in the n- and p-type cladding layers.
A semiconductor laser diode typically includes a body of a semiconductor material or materials having a waveguide region and a cladding region on each side of the waveguide region. Within the waveguide region is another region, such as a quantum-well region, in which photons are generated when the diode is properly biased by an electrical current. Generally, cladding regions are doped to be of opposite conductivity type from each other, and are of a material having a lower refractive index than the material of the waveguide region so as to attempt to confine the photons to the waveguide region.
Many applications of semiconductor laser diodes benefit from a broad waveguide laser design in which the waveguide thickness W is 3-4 times larger than W0, the waveguide thickness in conventional lasers that provides the maximum overlap of the optical zeroth-order mode with the quantum well region. As disclosed in U.S. Pat. No. 5,818,860, incorporated herein by reference, use of a broad waveguide can minimize overlap of the optical zeroth-order mode with the cladding layer. Waveguide broadening has the dual benefits of maximizing the laser""s power output due to decreased absorption in the cladding, and decreasing the probability of catastrophic optical damage of the mirror facet due to near field expansion.
Many applications, in addition to benefiting from a broad waveguide, require single-mode output. For example, semiconductor diode lasers are often used as a pumping source for signal amplification in fiber-optic telecommunication. When the signal amplification relies on erbium doped fiber amplifiers (EDFA), a single-mode laser output has high efficiency of coupling into the amplifier.
The present inventors have found that broadening the waveguide region above a certain material-dependent threshold (e.g., approximately 1 xcexcm for the materials used by the inventors) can lead to generation of additional optical modes, thereby decreasing the coupling of the laser output into the amplifier. Thus, known diode-laser configurations are limited in both near-field expansion and in vertical (perpendicular to the structure plane) far-field beam divergence narrowing. The former can cause damage to the laser facet when the laser is operated at a desired power, and the latter can decrease the efficiency of coupling the laser output into other devices
To alleviate the problems in known systems, a high-power semiconductor diode laser having a quantum well or multiple quantum wells substantially in the waveguide center is discussed herein.
Embodiments of the present invention allow for broadening the waveguide beyond that in previously-designed lasers, while maintaining the laser""s single-mode properties with improved near-field distribution and far-field divergence. To achieve these results, various embodiments are configured such the refractive-index distribution is asymmetric as measured from the cladding on one side of the waveguide region to the cladding on the other side of the waveguide region (i.e., in the vertical direction). Additionally, embodiments of the present invention include various configurations in which a quantum well is positioned in the center of the waveguide region.
The asymmetry of the refractive index distribution prevents non-zero even modes from lasing in the waveguide region, while the central position of the quantum well prevents odd-modes from lasing in the waveguide region. For the purposes of the present invention, the phrases xe2x80x9cnon-zero even modesxe2x80x9d and xe2x80x9codd modesxe2x80x9d represent other higher-order modes excluding the fundamental mode.
The vertical design of the laser can, in one embodiment, provide a far-field beam divergence of approximately 20 degrees, which will readily couple into an EDFA with a core diameter of 4 xcexcm having a numerical aperture (NA) of 0.2.
In one embodiment of the present invention, a single-mode semiconductor diode laser includes a waveguide region, a p-type cladding layer and an n-type cladding layer. The waveguide region has a first refractive index n1, the p-type cladding layer has a second refractive index n2 smaller than the first refractive index, and the n-type cladding layer has a third refractive index n3 smaller than the first refractive index and larger than the second refractive index. The bandgap difference between the p-type cladding layer and the waveguide region is larger than that between the n-type cladding layer and the waveguide.